This invention is directed to external cavity semiconductor lasers.
Tunable external cavity diode lasers (ECDLs) are widely used in light-based test and measurement equipment and are increasingly used in wavelength division multiplexed (WDM) optical voice and data communications devices. Tunable external cavity diode lasers operate by having a laser gain medium inserted into an external cavity, which provides wavelength-selective optical feedback to the laser gain medium.
Multiple longitudinal spectral modes may be supported by the cavity, corresponding to wavelengths such that the optical cavity length is an integral number of half-wavelengths. The gain medium provides nearly equivalent gain at the wavelength of the adjacent modes so mode hopping may occur, when, while changing the tuning of the external cavity, an adjacent external cavity longitudinal mode experiences a similar external cavity loss. The cavity may support the multiple modes, or may “hop” between the modes due to mode competition.
Tuning of the external cavity may generally be achieved by inserting a wavelength selector in the cavity, such as a diffraction grating. The diffraction grating may be used as the end reflector in the external cavity, in the “Littrow” arrangement, or may be disposed before the end reflector in the “Littman” arrangement. In the Littman arrangement, the laser beam is incident on the grating at a grazing incidence, and the first order diffracted beam is reflected from an end tuning mirror. The end tuning mirror then retroreflects the beam of the selected wavelength back onto the grating and into the laser diode. The wavelength may be tuned by changing the angle of the end tuning mirror.
In practice, the Littman arrangement may be preferred, because of the large area of the grating covered by the laser beam at grazing incidence, and because the double pass of the beam on the wavelength-selecting grating results in higher spectral selectivity.
In either the Littman or Littrow arrangements, in order to obtain a continuous tuning of the output of the laser diode that is free of mode hops, the cavity length must be changed in a synchronized manner with the wavelength tuning of the external cavity, so that the same integral number of half wavelengths is maintained within the cavity during tuning. However, the total cavity length, that is, the effective optical path length, must also take into account variations in the cavity length due to chromatic dispersion of the various materials within the cavity. Chromatically dispersive elements are elements having an index of refraction which depends on the wavelength of the light being transmitted through the material. Therefore, the effective optical path length of the external cavity depends on the wavelength of the light circulating in the external cavity.
Recently, various compact systems have been described that provide continuous mode-hop-free tuning for external cavity semiconductor lasers. Examples of these systems include U.S. Pat. No. 5,867,512 to Sacher (the '512 patent), and U.S. Pat. No. 5,319,668 to Luecke (the '668 patent), each of which is incorporated herein by reference in its entirety.
The '512 patent discloses a tuning arrangement for a semiconductor laser diode in an external resonator in a Littman arrangement, with a diffraction grating and a resonator end mirror mounted on a pivotable tuning arm. An adjustment means is provided which adjusts the position of the tuning arm and the location of the end mirror relative to the pivot axis of the tuning arm. Another adjustment means may be provided to adjust the location of the diffraction grating relative to the pivot axis of the tuning arm.
The '668 patent discloses a tunable laser system in which the pivot point of the pivotable end mirror may be selected so as to provide an internal cavity length which is exactly an integral number of half wavelengths at three different wavelengths and a close match at all other wavelengths within the tuning range. The pivot point calculation takes into account the effect of dispersion of the laser and other optical elements in the system on the cavity length. The pivot bearing in the '668 patent is a ball that rests in a socket.